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Re: (ITS#8424) mdb_env_cwalk() is broken, leading to crash if MDB_CP_COMPACT is set

ddmitrie@gmail.com wrote:
> Full_Name: Dmitry Dmitrienko
> Version: trunk, 0.9.18
> OS: Linux, Windows
> URL:
> Submission from: (NULL) (
Thanks, fixed now in git mdb.master

> In short:
> Reproduce: create a large database (1GB would be enough), run md_copy with -c
> argument. Expected result - compacted new database, Actual result - empty new
> database and crash in md_copy.
> In details:
> mdb_env_cwalk() defines cursor and does not initialize most of its fields,
> including mc_flags field. Then it calls mdb_page_get() procedure, which checks
> if (C_ORIG_RDONLY|C_WRITEMAP) flags are set in mc_flags. Since this field is not
> initialized, it may have arbitrary value. If these flags are not set, it starts
> checking txn's mt_u.dirty_list, as if it was a writing transaction, while in
> fact this is a walker, it is a reader, so it initialized mt_u.reader fields.
> This field is in the same memory location with dirty_list just because it's
> union. So mdb_page_get() actually reads lock file memory (it's where mt_u.reader
> points to). If the database is large enough, it leads to access to far beyond
> lock file memory mapped region and crashes md_copy.
> Proposed fix:
>   mdb_env_cwalk(mdb_copy *my, pgno_t *pg, int flags)
>   ...
> +	memset(&mc, 0, sizeof(mc));
> 	mc.mc_snum = 1;
> 	mc.mc_top = 0;
> 	mc.mc_txn = my->mc_txn;
> +	mc.mc.mc_flags = my->mc_txn->mt_flags & (C_ORIG_RDONLY|C_WRITEMAP);

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